Process and apparatus for separation of hydrocarbons and nitrogen

ABSTRACT

The invention provides a process and apparatus for the separation of nitrogen from a gaseous feed comprising a mixture of hydrocarbons and nitrogen gas, the process comprising the steps of: (i) cooling and at least partially condensing the gaseous feed; (ii) feeding the cooled and at least partially condensed feed from step (i) to a first fractionation to produce an overhead vapour stream having an enriched nitrogen content and a condensed product having a reduced nitrogen content which is subjected to a second fractionation, which comprises reboil, at a lower pressure than the first fractionation; (iii) partially condensing the overhead vapour stream, and separating to provide a liquid stream, which is used to provide reflux to the first fractionation, and a separated vapour stream, which is condensed to provide reflux to the second fractionation; and (iv) sub-cooling the condensed product of the first fractionation and dividing the resulting sub-cooled product into at least two streams: a first stream being expanded and fed to the second fractionation, and a second stream being expanded and reheated in heat exchange with the separated vapour stream from step (ii) before being fed to the second fractionation; (v) removing a hydrocarbon product stream low in nitrogen from the second fractionation; and (vi) removing a nitrogen rich stream from the second fractionation.

This invention relates to processes and apparatus for the lowtemperature separation of nitrogen from a gaseous mixture comprisingnitrogen gas and hydrocarbons. Such mixtures occur naturally ingeological formations and can also result from nitrogen injection as amethod of improving oil or gas production. Nitrogen separation may berequired as part of an overall processing of gaseous hydrocarbons tomeet sales specifications, such as maximum inert content or minimumcalorific value.

Low temperature fractionation presents an energy efficient method forthe separation of nitrogen from gaseous hydrocarbon streams, inparticular gaseous hydrocarbon streams wherein the hydrocarbons comprisepredominantly methane, such as natural gas. Separated nitrogen streamsof high purity can be produced, thereby maximising hydrocarbon recoveryand, where the nitrogen stream is vented to atmosphere, minimisingenvironmental impact.

Where the nitrogen content is higher than approximately 35 to 40 mol %,a double column arrangement (such as disclosed in U.S. Pat. No.7,127,915) similar to that used in air separation is conventional and isoften the most economical choice considering both capital cost andenergy consumption. The columns are typically configured in a stackedarrangement, with the upper fractionation column operating at lowpressure, just above atmospheric, and the lower fractionation columnoperating at high pressure, typically at approximately 27 bar (2700kPa).

An example of a conventional double column arrangement is shown in FIG.1.

A feed gas (01) is cooled and at least partially condensed in heatexchanger (02). The partially condensed feed (03) is expanded acrossvalve (06) to form a two-phase feed (07) which is fed to a high pressurecolumn (08). The high pressure column (08) separates the two-phase feed(07) into a nitrogen rich overhead vapour fraction (19) and ahydrocarbon rich liquid fraction (09).

The hydrocarbon rich liquid fraction (09) from the high pressure column(08) is sub-cooled in heat exchanger (10) to form stream (12) which isexpanded across valve (13) to form a further two-phase feed stream (14)which is fed to an intermediate stage of a low pressure column (15).

The overhead vapour (19) from the high pressure column (08) is fullycondensed in heat exchange with boiling liquid at the bottom of the lowpressure column (15) in reboil heat exchanger (04). The liquid mayeither be piped from the bottom tray or packed section, or the reboilheat exchanger (04) may be submerged in the liquid in the sump of thelow pressure column (15).

The fully condensed overhead stream (20) is split. A portion is passedas reflux (22) to the high pressure column (08) and a portion (23) issub-cooled in heat exchangers (10) and (24) to form stream (25) which isexpanded across valve (26) and passes as reflux (27) to the low pressurecolumn (15).

A hydrocarbon product (31) with low nitrogen content, from the lowpressure column (15), is pumped to an elevated pressure, dependent onthe composition and pressure of feed gas (01), and is rewarmed andevaporated in heat exchangers (10) and (02) to form a gaseous product(35), providing the majority of the refrigeration for cooling andcondensation of the feed gas (01).

A nitrogen vapour stream (28) with low methane content from the lowpressure column (15) is rewarmed in heat exchanger (24) to providerefrigeration for sub-cooling of the nitrogen rich reflux stream and isfurther rewarmed in heat exchangers (10) and (02).

The purity of the hydrocarbon product (31) from the bottom of the lowpressure fractionation column (15) is ensured by provision of sufficientreboil in heat exchange with the overhead vapour from the high pressurecolumn (08) in the reboil heat exchanger (04). Purity of the nitrogenproduct from the low pressure fractionation column overhead (28) isensured by conditioning the feed streams to the column and, inparticular, providing sufficient flow of nitrogen rich reflux (27).

The pressure drop between high pressure fractionation column (08) andlow pressure fractionation column (15) provides the requiredrefrigeration by Joule-Thomson expansion. High purity nitrogen andhydrocarbon streams are withdrawn from the top and bottom of the lowpressure fractionation column (15) respectively.

When the nitrogen content of the feed gas to a conventional doublecolumn system is lower than approximately 35 mol %, insufficient refluxis normally generated to the low pressure fractionation column tomaintain low losses of hydrocarbons with the nitrogen product.

In such cases, options to increase the available reflux include:

-   a) compressing the reject nitrogen stream and recycling it to the    feed gas or other part of the process to meet reflux requirements;    and-   b) introducing an upstream ‘pre-separation’ system to condition the    feed gas to produce a stream suitably enriched in nitrogen to feed    to the double column system.

Process selection will be dependent on considerations such as plantcapacity, feed gas nitrogen content and variability and feed and productgas pressure. Use of an upstream ‘pre-separation’ system is efficient inproducing product gas at elevated pressure, hence reducing product gascompression power requirements and may also be preferred when thegaseous feed comprises contaminants such as carbon dioxide and heavyhydrocarbons, which are tolerated at higher levels in the pre-separationsystem. Inclusion of either a pre-separation system or a nitrogenrecycle system does however introduce additional equipment items.

The present invention provides an alternative double column system forthe separation of nitrogen from a gaseous mixture comprising nitrogengas and hydrocarbons which uses improved heat integration to maximisethe nitrogen reflux stream and to optimise the feed conditions to thelow pressure fractionation column. This provides additional flexibilityto process gases having low nitrogen content, for example less than 35mol %, and potentially as low as 20 mol % without recourse to the use ofadditional equipment for pre-separation or nitrogen recycling asdescribed above. However, the invention is also useful for theseparation of gases having nitrogen content above 35 mol %, withimproved separation efficiency being observed in comparison withconventional separation techniques.

The present invention provides a process for the separation of nitrogenfrom a gaseous feed comprising a mixture of hydrocarbons and nitrogengas, the process comprising the steps of:

-   -   (i) cooling and at least partially condensing the gaseous feed;    -   (ii) feeding the cooled and at least partially condensed feed        from step (i) to a first fractionation to produce an overhead        vapour stream having an enriched nitrogen content and a        condensed product having a reduced nitrogen content which is        subjected to a second fractionation, which comprises reboil, at        a lower pressure than the first fractionation;    -   (iii) partially condensing the overhead vapour stream, and        separating to provide a liquid stream, which is used to provide        reflux to the first fractionation, and a separated vapour        stream, which is condensed to provide reflux to the second        fractionation; and    -   (iv) sub-cooling the condensed product of the first        fractionation and dividing the resulting sub-cooled product into        at least two streams: a first stream being expanded and fed to        the second fractionation, and a second stream being expanded and        reheated in heat exchange with the separated vapour stream from        step (iii) before being fed to the second fractionation;    -   (v) removing a hydrocarbon product stream low in nitrogen from        the second fractionation; and    -   (vi) removing a nitrogen rich stream from the second        fractionation.

The combination of steps (iii) and (iv) enables separation of feedstreams with lower nitrogen content than is feasible with prior artprocesses, such as the conventional double column arrangement describedabove.

In particular, partial condensation of the overhead vapour stream fromthe first fractionation allows the separated vapour stream from step(iii) to be efficiently condensed in heat exchange with reheat of thesecond stream of the condensed product in step (iv) to provide reflux tothe second fractionation. The reheated second stream has an increasedvapour fraction and its introduction into the second fractionationprovides an increased quantity of stripping vapour to the secondfractionation. For a given feed composition, the effect of these stepsis to provide improved separation, even for feed compositions comprisingless than 35 mol % nitrogen, for example 20 to 35 mol %, whilemaintaining the overall energy efficiency of the separation process.

As a further advantage, partial condensation of the overhead stream fromthe first fractionation allows the first fractionation and the secondfractionation steps to be ‘decoupled’ as there is no physicalrequirement for the second fractionator to be elevated above the firstfractionator.

In a preferred embodiment, the hydrocarbons in the gaseous feed compriseor consist of methane. For instance, the gaseous feed may comprise orconsist of natural gas. In further preferred embodiments, the gaseousfeed comprises less than 40 mol % nitrogen, less than 35 mol % nitrogen,or less than 30 mol % nitrogen. Preferably, the gaseous feed comprisesat least 20 mol % nitrogen. The gaseous feed may further comprise otherinert gases, such as helium. If required, the gaseous feed may besubjected to one or more pre-treatment procedures to remove impuritiesand/or unwanted components which could solidify in either of thefractionations.

The gaseous feed is cooled and partially condensed prior to the firstfractionation. In order to minimise power consumption, heat exchangeduring cooling of the gaseous feed may be used to provide reboil to thesecond fractionation. In addition, cooling of the gaseous feed may beobtained by heat exchange with the hydrocarbon product low in nitrogenfrom the second fractionation and/or the nitrogen rich stream from thesecond fractionation. In one preferred embodiment, the hydrocarbonproduct low in nitrogen from the second fractionation is pumped toelevated pressure and evaporated to provide cooling to the gaseous feed.

The gaseous feed may also be split into at least two streams, each ofwhich may be independently processed according to any of the stepsdescribed above before being directed to the first fractionation. Whereat least one of the at least two streams is used to provide reboil tothe second fractionation energy efficiently via heat exchange, thatstream is preferably fed to an intermediate stage of the firstfractionation. Where at least one of the at least two streams is notused to provide reboil to the second fractionation, that stream ispreferably fed below the bottom stage of the first fractionation toprovide stripping vapour. Preferably the first fractionation is at apressure of 5 to 30 bar (0.5 to 3.0 MPa).

In addition, reboil in the second fractionation may be provided by heatexchange with the overhead vapour stream from the first fractionationduring the partial condensation thereof, thereby reducing energyconsumption.

In step (iv) of the process described above, the first stream maycomprise between 10% and 50% of the sub cooled product.

In a preferred embodiment, the first stream is expanded to form atwo-phase feed, prior to being fed to the second fractionation. Thesecond stream is preferably expanded to form a two-phase feed beforebeing reheated via heat exchange with the separated vapour stream fromstep (iii). If required, further reheating of the second stream, afterexpansion thereof, may be effected by heat exchange with the condensedproduct from the first fractionation and/or the overhead vapour streamfrom the first fractionation. Preferably the first stream is fed to thesecond fractionation at a higher stage than the second stream.

The hydrocarbon product stream low in nitrogen from the secondfractionation is preferably removed from the second fractionation as aliquid stream.

In a further embodiment, the nitrogen rich stream from the secondfractionation may be reheated by heat exchange with the overhead vapourstream from the first fractionation during partial condensation thereofand/or by heat exchange with the separated vapour stream in step (iii)during condensation thereof.

It will be appreciated by the skilled person that the residual nitrogencontent of the hydrocarbon product and the residual hydrocarbon contentof the nitrogen rich stream obtained from the second fractionation aredependent on the composition of the feed gas. However, the process ofthe present invention typically provides a hydrocarbon productcomprising 2 mol % or less residual nitrogen content, and possibly ahydrocarbon product comprising less than 1 mol % residual nitrogen canbe obtained. However, in other embodiments the process may be operatedwith a more relaxed specification so as to obtain a hydrocarbon producthaving, for example, up to 10 mol % residual nitrogen content.

The present invention also provides an apparatus for the separation ofnitrogen from a gaseous feed comprising a mixture of hydrocarbons, theapparatus comprising of:

-   -   (i) means for cooling and at least partially condensing the        gaseous feed;    -   (ii) a first fractionator for producing an overhead vapour        stream and a condensed product and a second fractionator        operable at a lower pressure than the first fractionator;    -   (iii) means for conveying the cooled and at least partially        condensed feed from step (i) to the first fractionator;    -   (iv) means for conveying the condensed product from the first        fractionator to the second fractionator;    -   (v) means for partially condensing the overhead vapour stream,        and means for separating the partially condensed vapour stream        to provide a liquid stream, and a vapour stream;    -   (vi) means for conveying the liquid stream to the first        fractionator which is used to provide reflux to the first        fractionation, and means for conveying and condensing the vapour        stream to provide reflux to the second fractionator;    -   (vii) means for dividing the condensed product of the first        fractionator, prior to entry into the second fractionator, into        at least two streams;    -   (viii) means for expanding a first stream prior to entry into        the second fractionator, and means for expanding and heating        prior to entry into the second fractionator;    -   (ix) means for conveying a hydrocarbon product low in nitrogen        from the second fractionator; and    -   (x) means for conveying a nitrogen rich stream from the second        fractionator.

In one embodiment, the first and second fractionators may be in astacked configuration, with the second fractionator positioned above thefirst fractionator.

In an alternate embodiment, the overall height of the apparatus may bereduced by arranging the first and second fractionators in a non-stackedconfiguration.

Preferably, the second fractionator comprises a reboil heat exchanger.

Suitable means for expanding the streams include liquid and two-phaseexpansion turbines.

The invention will now be described in greater detail with reference topreferred embodiments and with the aid of the accompanying figures, inwhich:

FIG. 1 shows a conventional stacked double column apparatus for theseparation of nitrogen from a gaseous mixture comprising nitrogen gasand hydrocarbons, as described above.

FIG. 2 shows a stacked double column apparatus in accordance with thepresent invention.

FIG. 3 shows an uncoupled double column apparatus also in accordancewith the present invention

FIG. 4 also shows an uncoupled double column apparatus in accordancewith the present invention.

In the embodiment of the invention shown in FIG. 2, a high pressurecolumn (08) and a low pressure column (15) are provided in a stackedarrangement, with the high pressure column (08) positioned below the lowpressure column (15).

A feed gas (01) is cooled and at least partially condensed in a heatexchanger (02) and is expanded across valve (06) to form two-phase feed(07) to the bottom of the high pressure column (08). The high pressurecolumn (08) separates the two-phase feed (07) into a nitrogen richoverhead vapour fraction (19) and a hydrocarbon rich liquid fraction(09)

The hydrocarbon rich liquid fraction (09) from the high pressure column(08) is sub-cooled in heat exchanger (10) and the resulting stream (12)is split into two portions. One portion is expanded across valve (13) toform a two-phase feed stream (14) which is fed to an intermediate stageof the low pressure column (15). The other portion is expanded acrossvalve (16) and is reheated at low pressure to form a two-phase feedstream (18), which has a higher vapour fraction and is fed to a lowerstage of the low pressure column (15) than feed stream (14).

The overhead vapour (19) from the high pressure column (08) is partiallycondensed in heat exchange with boiling liquid at the bottom of the lowpressure column (15) in a reboil heat exchanger (04). The boiling liquidmay either be piped to the reboil heat exchanger (04) from a bottom trayor packed section of the low pressure column (15), or the reboil heatexchanger (04) may be submerged in the liquid in the sump of the lowpressure column (15).

The partially condensed overhead stream (20) is separated into a liquidstream (22) and a separated vapour stream (23) in a phase separator(21). The liquid stream (22) is passed as reflux to the high pressurecolumn (08). The separated vapour stream (23) is fully condensed andsub-cooled in heat exchangers (10) and (24) to form stream (25) which isexpanded across valve (26) and is passed as reflux (27) to the lowpressure column (15).

A hydrocarbon product (31) with low nitrogen content from the lowpressure column (15) is pumped to an elevated pressure by a pump (32),dependent on the composition and pressure of the feed gas (01), and theresulting stream (33) is evaporated and reheated in heat exchangers (10)and (02) to form a gaseous product (35). Evaporation and reheating ofthe hydrocarbon stream (33) in heat exchanger (02) preferably providesat least a portion of, and more preferably the majority of, therefrigeration required for cooling and condensation of the feed gas(01).

A nitrogen vapour stream (28) with low hydrocarbon content from the lowpressure column (15) is preferably reheated in heat exchanger (24) toprovide further refrigeration for sub-cooling of the separated vapourstream (23) and is preferably further reheated in heat exchangers (10)and (02).

In the embodiment of the invention shown in FIG. 3, a high pressurecolumn (08) and a low pressure column (15) are provided in an uncoupledarrangement.

A feed gas (01) is cooled and at least partially condensed in a heatexchanger (02) and is then sub-cooled in heat exchange with boilingliquid at the bottom of the low pressure column (15) in a reboil heatexchanger (04). The boiling liquid may either be piped to the reboilheat exchanger (04) from a bottom tray or packed section of the lowpressure column (15), or the reboil heat exchanger (04) may be submergedin the liquid in the sump of the low pressure column (15). The cooledand at least partially condensed feed gas (05) is expanded across valve(06) to form two-phase feed (07) to the bottom of the high pressurecolumn (08).

The hydrocarbon rich liquid fraction (09) from the high pressure column(08) is sub-cooled in heat exchangers (10) and (11) and the resultingstream (12) is split into two portions. One portion is expanded across avalve (13) to form a two-phase feed stream (14) to an intermediate stageof the low pressure column (15). The other portion is expanded across avalve (16) and is reheated at low pressure to form a two-phase feedstream (18), which has a higher vapour fraction and is fed to a lowerstage of the low pressure column (15) than feed stream (14).

The overhead vapour (19) from the high pressure column (08) is partiallycondensed in heat exchanger (10). The partially condensed overheadstream (20) is separated into a liquid stream (22) and a separatedvapour stream (23) in a phase separator (21). The liquid stream (22) ispassed as reflux to the high pressure column (08). The separated vapourstream (23) is fully condensed and sub-cooled in heat exchangers (11)and (24) to form stream (25) which is expanded across a valve (26) andis passed as reflux (27) to the low pressure column (15).

A hydrocarbon product (31) with low nitrogen content, from the lowpressure column (15), is pumped to an elevated pressure by a pump (32),dependent on the composition and pressure of the feed gas (01), and theresulting stream (33) is evaporated and reheated in heat exchanger (02)to form a gaseous product (34). Evaporation and reheating of thehydrocarbon stream (33) in the heat exchanger (02) preferably providesat least a portion of, and more preferably the majority of, therefrigeration required for cooling and condensation of the feed gas(01).

A nitrogen vapour stream (28) with low hydrocarbon content from the lowpressure column (15) is preferably reheated in heat exchanger (24) toprovide further refrigeration for sub-cooling of the separated vapourstream (23) and is preferably further reheated in heat exchangers (10)and (02).

In the embodiment of the invention shown in FIG. 4, a high pressurecolumn (08) and a low pressure column (15) are provided in an uncoupledarrangement, and multiple feeds are provided to the high pressurecolumn.

A feed gas (01) is cooled and at least partially condensed in a heatexchanger (02) and is split with a portion being expanded across valve(36) to form a two-phase feed (37) which is fed to the bottom of highpressure column (08). The remaining portion of the feed gas (01) isfurther cooled in a heat exchanger (35) to form a stream (03) which isthen sub-cooled in heat exchange with boiling liquid at the bottom ofthe low pressure column (15) in a reboil heat exchanger (04). Theboiling liquid may either be piped to reboil heat exchanger (04) from abottom tray or packed section of the low pressure column (15), or thereboil heat exchanger (04) may be submerged in the liquid in the sump ofthe low pressure column (15). The cooled and at least partiallycondensed feed gas (05) is expanded across valve (06) to form feedstream (07) which is fed to an intermediate stage of high pressurecolumn (08).

The hydrocarbon rich liquid fraction (09) from the high pressure column(08) is sub-cooled in heat exchangers (10) and (11) and the resultingstream (12) is split into two portions. One portion is expanded acrossvalve (13) to form a two-phase feed stream (14) which is fed to anintermediate stage of a low pressure column (15). The other portion isexpanded across valve (16) and is reheated at low pressure to form atwo-phase feed stream (18) which has a higher vapour fraction and is fedto a lower stage of the low pressure column (15) than feed stream (14).

The overhead vapour (19) from the high pressure column is partiallycondensed in a heat exchanger (10). The partially condensed overheadstream (20) is separated into a liquid stream (22) and a separatedvapour portion (23) in a phase separator (21). The liquid stream (22)passes as reflux to the high pressure column (08). The separated vapourstream (23) is fully condensed and subcooled in heat exchangers (11) and(24) to form a stream (25) which is expanded across valve (26) and ispassed as reflux (27) to the low pressure column (15).

A hydrocarbon product (31) with low nitrogen content, from the lowpressure column (15), is pumped to an elevated pressure by pump (32),dependent on the composition and pressure of feed gas (01), and theresulting stream (33) is evaporated and reheated in heat exchangers (35)and (02) to form a gaseous product (34). Evaporation and reheating ofthe hydrocarbon stream (33) in the heat exchangers (35) and (02)preferably provides at least a portion of, and more preferably themajority of, the refrigeration required for cooling and condensation ofthe feed gas (01).

A nitrogen vapour stream (28) with low hydrocarbon content from the lowpressure column (15) is preferably reheated in heat exchanger (24) toprovide further refrigeration for sub-cooling of the separated vapourstream (23) and is preferably further reheated in heat exchangers (11),(10), (35) and (02).

EXAMPLES Comparative Example 1

Table 1 shows typical operating parameters for the conventional doublecolumn apparatus shown in FIG. 1 when used to separate a gaseous mixtureconsisting of 40 mol % nitrogen gas and 60 mol % methane. It will beobserved that, based on 6 theoretical separation stages in the highpressure column (08) and 6 theoretical separation stages in the lowpressure column (15), the conventional double column apparatus is ableto separate such a mixture to obtain a nitrogen product stream (Stream30) having a residual methane content of 0.8 mol % when producing amethane product stream (Stream 35) having a residual nitrogen content of2.0 mol %.

TABLE 1 Stream¹ 1 7 9 14 19 22 23 Pressure² MPa 3.00 2.70 2.70 0.22 2.682.68 2.68 Temperature ° C. 35.0 −127.6 −127.7 −171.2 −146.4 −149.6−149.6 Mass Flow kg/h 104155 104155 84014 84014 53792 33651 20140 MolarFlow mol/h 5000.0 5000.0 4255.5 4255.5 1988.2 1243.8 744.4 Nitrogen mol% 40.0 40.0 30.9 30.9 92.0 92.0 92.0 Methane mol % 60.0 60.0 69.1 69.18.0 8.0 8.0 Stream¹ 27 28 30 31 35 Pressure² MPa 0.22 0.20 0.15 0.220.98 Temperature ° C. −187.7 −188.3 28.5 −155.9 28.5 Mass Flow kg/h20140 54562 54562 49592 49592 Molar Flow mol/h 744.4 1954.4 1954.43045.6 3045.6 Nitrogen mol % 92.0 99.2 99.2 2.0 2.0 Methane mol % 8.00.8 0.8 98.0 98.0 ¹As identified in FIG. 1 ²Pressures are given asabsolute values

Comparative Example 2

Table 2 shows typical operating parameters for the conventional doublecolumn apparatus shown in FIG. 1 when used to separate a gaseous mixtureconsisting of 30 mol % nitrogen gas and 70 mol % methane. It will beobserved that, based on the same number of theoretical separation stagesin columns (08) and (15) as per Comparative Example 1, and producing amethane product stream (Stream 35) having a residual nitrogen content of2.0 mol %, hydrocarbon recovery in the conventional double columnapparatus is reduced, with the residual methane content of the nitrogenproduct stream (Stream 30) increasing to 11.1 mol %.

TABLE 2 Stream¹ 1 7 9 14 19 22 23 Pressure² MPa 3.00 2.70 2.70 0.22 2.682.68 2.68 Temperature ° C. 35.0 −122.5 −122.6 −167.7 −147.1 −149.9−149.9 Mass Flow kg/h 98170 98170 85562 85562 45074 32466 12608 MolarFlow mol/h 5000.0 5000.0 4536.0 1536.0 1658.6 1194.6 464.0 Nitrogen mol% 30.0 30.0 23.6 69.6 93.0 93.0 93.0 Methane mol % 70.0 70.0 76.4 30.47.0 7.0 7.0 Stream¹ 27 28 30 31 35 Pressure² MPa 0.22 0.20 0.15 0.220.12 Temperature ° C. −187.7 −176.4 29.3 −155.9 29.3 Mass Flow kg/h12608 42981 42981 55188 55188 Molar Flow mol/h 464.0 1610.5 1610.53389.5 3389.5 Nitrogen mol % 93.0 88.9 88.9 2.0 2.0 Methane mol % 7.011.1 11.1 98.0 98.0 ¹As identified in FIG. 1 ²Pressures are given asabsolute values

Example 3

Table 3 shows typical operating parameters for the process of theinvention using the apparatus shown in FIG. 2, when used to separate agaseous mixture consisting of 40 mol % nitrogen gas and 60 mol %methane. With this gaseous mixture, the first stream accounts for 33% bymolar flow of the sub cooled product. It will be observed that, based onthe same number of theoretical separation stages in columns (08) and(15) as per Comparative Example 1, the process of the invention is ableto separate such a mixture to obtain a nitrogen product stream (Stream30) having an improved residual methane content of 0.4 mol % whenproducing a methane product stream (Stream 35) having a residualnitrogen content of 2.0 mol %.

TABLE 3 Stream¹ 1 7 9 14 18 19 22 Pressure² MPa 3.00 2.50 2.50 0.22 0.222.48 2.48 Temperature ° C. 35.0 −129.8 −129.9 −178.9 −158.2 −149.1−151.3 Mass Flow kg/h 104155 104155 84366 56508 27858 53384 33596 MolarFlow mol/h 5000.0 5000.0 4284.0 2869.4 1414.6 1957.7 1241.7 Nitrogen mol% 40.0 40.0 30.5 30.5 30.5 93.8 92.0 Methane mol % 60.0 60.0 69.5 69.569.5 6.2 8.0 Stream¹ 23 27 28 30 31 35 Pressure² MPa 2.48 0.22 0.20 0.150.22 0.98 Temperature ° C. −151.3 −188.2 −188.9 28.5 −155.9 28.5 MassFlow kg/h 19789 19789 54443 54443 49712 49712 Molar Flow mol/h 716.0716.0 1946.7 1946.7 3053.3 3053 Nitrogen mol % 96.9 96.9 99.6 99.6 2.02.0 Methane mol % 3.1 3.1 0.4 0.4 98.0 98.0 ¹As identified in FIG. 2²Pressures are given as absolute values

Example 4

Table 4 shows typical operating parameters for the process of theinvention using the apparatus shown in FIG. 2, when used to separate agaseous mixture consisting of 30 mol % nitrogen gas and 70 mol %methane. In contrast with the conventional double column apparatus shownin FIG. 1, the process and apparatus of the invention is able tomaintain good separation efficiency even when the nitrogen content ofthe gaseous feed is below 35 mol %. With this gaseous mixture, the firststream accounts for 25% by molar flow of the sub cooled product. Thus,using the process and apparatus of the invention, based on the samenumber of theoretical separation stages in columns (08) and (15) as perComparative Example 2, it is possible to obtain a nitrogen productstream (Stream 30) having a residual methane content of 1.0 mol %(compared with 11.1 mol % in Comparative Example 2), when producing amethane product stream (Stream 35) having a residual nitrogen content of2.0 mol %.

TABLE 4 Stream¹ 1 7 9 14 18 19 22 Pressure² MPa 3.00 2.00 2.00 0.22 0.221.98 1.98 Temperature ° C. 35.0 −131.7 −131.7 −175.6 −156.3 −146.9−153.2 Mass Flow kg/h 98170 98170 84871 63762 21109 28263 14964 MolarFlow mol/h 5000.0 5000.0 4513.1 3390.7 1122.5 1071.0 584.1 Nitrogen mol% 30.0 30.0 23.1 23.1 23.1 86.4 80.0 Methane mol % 70.0 70.0 76.9 76.976.9 13.6 20.0 Stream¹ 23 27 28 30 31 35 Pressure² MPa 1.98 0.22 0.200.15 0.22 1.18 Temperature ° C. −153.2 −187.9 −188.0 29.3 −155.9 29.3Mass Flow kg/h 13299 13299 40267 40267 57903 57903 Molar Flow mol/h486.9 486.9 1443.7 1443.7 3556.3 3556.3 Nitrogen mol % 94.2 94.2 99.099.0 2.0 2.0 Methane mol % 5.8 5.8 1.0 1.0 98.0 98.0 ¹As identified inFIG. 2 ²Pressures are given as absolute values

1. A process for the separation of nitrogen from a gaseous feedcomprising a mixture of hydrocarbons and nitrogen gas, the processcomprising the steps of: (i) cooling and at least partially condensingthe gaseous feed; (ii) feeding the cooled and at least partiallycondensed feed from step (i) to a first fractionation to produce anoverhead vapour stream having an enriched nitrogen content and acondensed product having a reduced nitrogen content which is subjectedto a second fractionation, which comprises reboil, at a lower pressurethan the first fractionation; (iii) partially condensing the overheadvapour stream, and separating to provide a liquid stream, which is usedto provide reflux to the first fractionation, and a separated vapourstream, which is condensed to provide reflux to the secondfractionation; and (iv) sub-cooling the condensed product of the firstfractionation and dividing the resulting sub-cooled product into atleast two streams: a first stream being expanded and fed to the secondfractionation, and a second stream being expanded and reheated in heatexchange with the separated vapour stream from step (ii) before beingfed to the second fractionation; (v) removing a hydrocarbon productstream low in nitrogen from the second fractionation; and (vi) removinga nitrogen rich stream from the second fractionation.
 2. A processaccording to claim 1, wherein the hydrocarbons in the gaseous feedcomprises methane.
 3. A process according to claim 1, wherein thegaseous feed comprises natural gas.
 4. A process according to claim 1,wherein the gaseous feed comprises less than 40 mol % nitrogen.
 5. Aprocess according to claim 4, wherein the gaseous feed comprises lessthan 35 mol % nitrogen.
 6. A process according to claim 5, wherein thegaseous feed comprises less than 30 mol % nitrogen.
 7. A processaccording to claim 1, wherein the gaseous feed comprises at least 20 mol% nitrogen.
 8. A process according to claim 1, wherein reboil in thesecond fractionation is provided at least in part by heat exchangeduring cooling of the gaseous feed.
 9. A process according to claim 1,wherein the gaseous feed is cooled and at least partially condensed viaheat exchange with the hydrocarbon product stream low in nitrogen fromthe second fractionation and/or the nitrogen rich stream from the secondfractionation.
 10. A process according to claim 9, wherein thehydrocarbon product stream low in nitrogen is pumped to elevatedpressure and evaporated to provide cooling for the gaseous feed.
 11. Aprocess according to claim 1, wherein the gaseous feed is expanded priorto the first fractionation.
 12. A process according to claim 1, whereinthe gaseous feed is split into at least two streams, and each stream isindependently processed.
 13. A process according to claim 12, whereinthe at least two streams are directed to the first fractionation.
 14. Aprocess according to claim 12, wherein reboil in the secondfractionation is provided at least in part by heat exchange with atleast one of the at least two streams.
 15. A process according to claim14, wherein the at least one stream used for heat exchange is fed to anintermediate stage of the first fractionation.
 16. A process accordingto claim 15, wherein at least one of the at least two streams is notused for heat exchange in the second fractionation and is fed to abottom stage of the first fractionation.
 17. A process according toclaim 1, wherein reboil in the second fractionation is provided at leastin part by heat exchange during the partial condensing of the overheadvapour stream.
 18. A process according to claim 17, wherein the heatexchange is between the overhead vapour stream and a liquid product ofthe second fractionation.
 19. A process according to claim 1, whereinthe first fractionation is at a pressure in the range of 5 to 30 bar(0.5 to 3.0 MPa).
 20. A process according to claim 1, wherein in step(iv) the first stream is expanded to form a two-phase feed for feedingto the second fractionation.
 21. A process according to claim 1, whereinin step (iv) the second stream is expanded to form a two-phase feed andreheated via heat exchange for feeding to the second fractionation. 22.A process according to claim 21, wherein the second stream is reheatedvia heat exchange with the condensed product from the firstfractionation and/or the overhead vapour stream from the firstfractionation.
 23. A process according to claim 1, wherein the firststream in step (iv) is fed to the second fractionation at a higher stagethan the second stream.
 24. A process according to claim 1, wherein thenitrogen rich stream from the second fractionation is reheated by heatexchange with the overhead vapour stream from step (ii) and/or theseparated vapour stream from step (iii).
 25. A process according toclaim 1, wherein the hydrocarbon product stream is removed as an atleast partly liquefied product.
 26. A process according to claim 1,wherein the gaseous feed additionally comprises further inert gases. 27.A process according to claim 26, wherein the gaseous feed compriseshelium.
 28. A process according to claim 1, wherein the gaseoushydrocarbon feed is pre-treated to remove impurities and/or otherunwanted components which solidify in the first and secondfractionations.
 29. An apparatus for the separation of nitrogen from agaseous feed comprising a mixture of hydrocarbons and nitrogen gas, theapparatus comprising of: (i) means for cooling and at least partiallycondensing the gaseous feed; (ii) a first fractionator for producing anoverhead vapour stream and a condensed product and a second fractionatoroperable at a lower pressure than the first fractionator; (iii) meansfor conveying the cooled and at least partially condensed feed from step(i) to the first fractionator; (iv) means for conveying the condensedproduct from the first fractionator to the second fractionator; (v)means for partially condensing the overhead vapour stream, and means forseparating the partially condensed vapour stream to provide a liquidstream, and a separated vapour stream; (vi) means for conveying theliquid stream to the first fractionator which is used to provide refluxto the first fractionation, and means for conveying and condensing theseparated vapour stream to provide reflux to the second fractionator;and (vii) means for dividing the condensed product of the firstfractionator, prior to entry into the second fractionator, into at leasttwo streams; (viii) means for expanding a first stream prior to entrythereof into the second fractionator, and means for expanding andreheating a second stream prior to entry thereof into the secondfractionator; (ix) means for conveying a hydrocarbon product low innitrogen from the second fractionator; and (x) means for conveying anitrogen rich stream from the second fractionator.
 30. An apparatusaccording to claim 29, wherein the first and second fractionators are ina stacked configuration.
 31. An apparatus according to claim 29, whereinthe first and second fractionators are in a non-stacked configuration,allowing reduction on the overall height of the apparatus.
 32. Anapparatus according to claim 29, wherein the second fractionatorcomprises a reboil heat exchanger.
 33. An apparatus according to claim29, wherein the means for expanding the streams comprises liquid ortwo-phase expansion turbines.
 34. An apparatus according to claim 29wherein the second stream in step (viii) is reheated in heat exchangewith the separated vapour stream from step (v).